1. Field of the Invention PA1 2. Description of the Related Art
The invention relates to a relief valve to be used for controlling a hydraulic pressure of a hydraulic circuit.
For example, a hydraulic motor is generally used for a turning motor and a traveling motor for a construction machine vehicle, and is driven by a hydraulic circuit. A relief valve is used for controlling a hydraulic pressure of the hydraulic circuit.
FIG. 8 shows an example of a hydraulic circuit using a relief valve. As shown, a relief valve R10 communicates with a pressurized liquid supply port and a pressurized liquid exhaust port of a hydraulic motor M. An escape side of the relief valve R10 communicates with a liquid tank T through a boost check valve B for keeping a suction (negative) pressure of the motor. P denotes a hydraulic pump and V denotes a switching valve.
FIG. 10 is a sectional view showing a relief valve disclosed in Japanese Utility Model Publication No. Hei 7-23663. In the relief valve, a plunger 103 is pressed forward by means of a coiled spring 105. The plunger 103 moves rearward against elastic force of the coiled spring 105 to make an inlet 102a and an outlet 101a communicate with each other when a pressure of the inlet 102a is raised. A rear end of the coiled spring 105 is pushed forward by means of a piston 104 inserted and fitted in an inner bore of a case 101 for sliding. The piston 104 moves forward to compress the coiled spring 105 when the pressure of the inlet 102a is raised. Thus, a relief pressure is regulated. The plunger 103 can slide along a central axis of the piston 104.
In the relief valve having the above-mentioned structure, however, an annular sectional area having a diameter d104 as an outside diameter and a diameter d103 as an inside diameter is an effective pressure receiving area for actuating the piston 104. When the annular sectional area is excessively large, the piston 104 is actuated at a low pressure. Consequently, the actuation of the piston 104 is completed before the pressure of the inlet 102a reaches the relief pressure. As a result, a surge pressure is generated before the pressure of the inlet 102a reaches the relief pressure.
FIG. 11A shows a temporal change of a pressure in the inlet 102a. In the above-mentioned relief valve, there is a possibility that a surge pressure might be generated.
If a meter-out pressure of a control valve is high, the piston 104 performs full stroke at the same pressure. As a result, as shown in FIG. 11B, pressure rise control is not performed at all in some cases.
It is sufficient to prevent the surge pressure from being generated that the annular sectional area is reduced. In such a manner, however, the diameter d104 should be caused to approximate to the diameter d103. Consequently, thickness of an annular portion of the piston 104 is desired to be reduced, but thereby strength of the annular portion is made insufficient.
Even if a pressure rise control time is to be increased, there is a limit, because a volume absorbed by the piston 104 is small. Although the pressure rise control time can be increased by means of giving damping to the piston 104c, there is also a limit to what can be done.